The invention relates to a bone screw having a screw body which is provided with an external thread and which comprises flow-through means for bone cement that is used for the production of a cement jacket around the bone screw.
Bone screws of this type are, for example, employed as anchoring screws in a stabilisation system when several bone elements have to be connected together. The formation of a cement jacket around the bone screw following the implantation process increases the stability of the anchorage. This is of particular importance when the bone is of reduced quality, such as is the case for osteoporosis of the bony tissue for example.
A bone screw comprising a longitudinal channel along which there is provided a plurality of radially extending transverse channels that are in contact with this longitudinal channel is known from EP 0 305 417 B1.
Based upon this prior art, the object of the invention is to provide a bone screw which, in comparison with the bone screws known from the prior art, will be anchored in a highly stable manner after being implanted in a bone, and especially after it has been implanted in bones of reduced quality.
In accordance with the invention, this object is achieved in the case of a bone screw of the type mentioned hereinabove, in that the flow-through means comprises at least one longitudinal recess which is formed in the screw body such that it extends transversely relative to the radial direction and transversely relative to the peripheral direction.
By virtue of a recess which is constructed and arranged in this manner and by means of which bone cement can be introduced into the intermediary space between an implanted bone screw and the bone bed, a uniform cement jacket can be formed around the screw. As a result of this feature, the bone screw in accordance with the invention will be anchored in a highly stable manner. Each of the areas between the turns of the external thread can be supplied with bone cement and a highly effective flow of bone cement can be achieved by means of such a longitudinal recess which is formed substantially along the length of the screw body. By contrast, in the case of the bone screw of EP 0 305 417 B1, an individual radial channel basically has to be provided for each of the intermediary spaces. Moreover, in order to also enable a cement jacket to be built-up in the region around the frontal end of the screw, the longitudinal channel used there must extend through the whole of the screw body. Construction of the screw in the form of a self-tapping screw is thereby adversely affected. By contrast, in the case of the bone screw in accordance with the invention, a longitudinal recess basically suffices, and a cement jacket can also be formed about the frontal end of the bone screw by means of appropriate recesses in the turns of the external thread.
It is expedient if the at least one longitudinal recess is arranged such that it is perpendicular to the radial direction and especially so, if it is arranged to be perpendicular to the peripheral direction.
It is particularly advantageous if the flow-through means comprise a hollow channel formed in the screw body and if a longitudinal recess is connected to the hollow channel. Consequently, bone cement introduced into the hollow channel can flow out directly from the body of the screw via such a longitudinal recess without encountering substantial barriers to the flow of material so as to form a uniform cement jacket around the bone screw. It is thereby ensured that the bone screw in accordance with the invention will be anchored in a highly stable manner after the implantation process. It is expedient if a longitudinal recess is connected to the hollow channel in the longitudinal direction of said recess so as to minimise the resistance encountered by the flow of bone cement.
It is particularly advantageous if the dimensions of a longitudinal recess in the longitudinal direction thereof are such that it extends over a plurality of turns of the external thread. Due to this feature, the bone cement can be supplied to the whole of the screw body using essentially just one longitudinal recess.
In a particularly advantageous embodiment, a longitudinal recess has corresponding recesses associated therewith in the turns of the external thread. This enables the bone cement to flow between different intermediate regions i.e. the regions between adjacent turns of the thread so as to enable the bone cement to be distributed uniformly and thus produce a uniform build-up of the cement around an implanted bone screw. It is expedient if a turn of the external thread is broken-through in the vicinity of a longitudinal recess so as to enable the bone cement to flow therethrough.
For manufacturing purposes, it is particularly expedient if a longitudinal recess is in the form of a slot. This feature enables the recess to be produced in a simple manner by a machining process, for example, a milling process. It is advantageous if a plane of the slot is substantially perpendicular to the peripheral direction of the screw body. In particular hereby, the slot is formed by an upper slot plane and a lower slot plane which are in parallel with one another. Due to the construction perpendicular to the peripheral direction, the slot plane can be oriented in the radial direction. This enables an efficient flow of bone cement when the screw body is rotationally symmetrical. Furthermore, it is also expedient if a plane of the slot is disposed substantially radially relative to the screw body.
It is particularly advantageous if a plane of the slot is located substantially longitudinally relative to the screw body. On the one hand, this ensures that such a longitudinal recess is easy to produce, and it also ensures an efficient flow of bone cement on the other.
For the purposes of forming a cement jacket which will ensure a high degree of stability for the anchorage, it is particularly advantageous if a plurality of longitudinal recesses are arranged over the periphery of the screw body. This feature ensures that the regions between the turns of the external thread taken with reference to the periphery of the screw body will be uniformly supplied with bone cement.
In one advantageous variant of this embodiment, three longitudinal recesses are arranged along the periphery of the screw body. On the one hand, this enables the number of break-throughs in the turns of the external thread and in the body of the screw to be kept low, and it also enables uniform application of the bone cement on the other.
Hereby, it is of especially great advantage if the plurality of longitudinal recesses are substantially symmetrical relative to a longitudinal axis of the screw body. In this manner, the bone cement, which is introduced into the body of the screw, flows uniformly via the longitudinal recesses into the regions between the turns of the external thread and forms a uniform cement jacket.
In a particularly advantageous embodiment, the bone screw in accordance with the invention comprises successive sections in the longitudinal direction of the screw body which differ in regard to the construction of the outer surfaces thereof. Due to these outer surfaces being constructed such as to have differing geometrical shapes, a particularly highly stable anchorage is attainable by, on the one hand, forming one or more sections which will ensure that the thread is held firmly in the bone and, on the other hand, forming sections which will exhibit a sealing effect in regard to the outward flow of the bone cement. In this manner, a cement jacket can be built-up which will increase the anchoring forces in every direction by virtue of it interlocking with the bone in the manner of meshing gears. The volume of bone cement that needs to be supplied can be precisely metered by appropriate selection of the dimensions of the sections. Consequently, problems associated with the heating effects produced by reactions in the cement are, to a great extent, avoided.
It is expedient if a frontal section of the screw body is substantially conical or is in the form of a truncated cone. An effective self-tapping function of the thread is thereby obtained. On the other hand, such a section is effective as a seal for preventing unintentional outflow of the bone cement.
Hereby, it is expedient if the envelope of the turns of the external thread is substantially conical or is in the form of a truncated cone in the frontal section thereof so as to obtain an effective self-tapping function for the thread. It is expedient if the envelope of the screw body is substantially conical or is in the form of a truncated cone in the frontal section thereof so as to produce a sealing effect for preventing unintentional outflow of the bone cement.
Furthermore, it is advantageous if a central section of the screw body is substantially cylindrical. The thread of an implanted screw will be held firmly in the bone by virtue of such a cylindrical section. Hereby, it is expedient if the envelope of the turns of the external thread in the central section is substantially cylindrical, and likewise, if the envelope of the screw body in the central section is substantially cylindrical.
It is of particularly great advantage if a hollow channel in the screw body is led through the central section, and especially if it is extended up to or close to the frontal section. The hollow channel does not then need to be continuous so that the self-tapping effect at the frontal end of the thread will not be adversely affected although, on the other hand, the bone screw can nevertheless still be uniformly coated with bone cement so as to form a cement jacket. For the same reasons, it is advantageous if the at least one longitudinal recess is arranged substantially in the central section.
It is expedient if a rear section of the screw body is substantially conical or is in the form of a truncated cone. By virtue of such a conical end region of the bone screw in accordance with the invention, a sealing effect for preventing unintentional outflow of bone cement from the bone bed in the case of an implanted bone screw can again be achieved.
Hereby, it is of particularly great advantage if the envelope of the screw body is substantially conical or is in the form of a truncated cone in the rear section so as to obtain the aforesaid sealing effect and if the envelope of the turns of the external thread is substantially cylindrical in the central section. Firstly hereby, one can achieve the sealing effect and secondly, the effect upon the bone caused by the boring action of the screw will not be increased thereby. It is expedient hereby if the envelope of the turns in the rear section and that of the turns in the central section coincide.
For production reasons, it is advantageous if the angle of the cone in the frontal section substantially corresponds to the angle of the cone in the rear section.
In order to form a uniform cement jacket around the bone screw and, in particular, to anchor the bone screw in accordance with the invention with great stability even in the case of bony tissue affected by osteoporosis, provision is made for the turns of the external thread to be provided with longitudinal breakthroughs in a region in which there are no longitudinal recesses. By virtue of this feature, substantially all of the regions surrounding the screw body can be supplied with bone cement since the bone cement is able to flow out from those intermediary spaces which are supplied via the longitudinal recesses and through the longitudinal breakthroughs into those regions that cannot be directly supplied. Thus, for example, a cement jacket can thereby be formed even round the frontal section of the bone screw in accordance with the invention.
It is quite particularly expedient and particularly easy for manufacturing purposes if a surface of the screw body between adjacent turns of the external thread is substantially parallel to the longitudinal direction of the screw body. A larger intermediary space is therefore made available for the bone cement, this thereby enabling, in particular, an effective flow of bone cement into the regions between the individual turns of the external thread for the purposes of distributing the cement around the periphery of the screw body.
For production purposes, it is particularly simple if the hollow channel is in the form of a blind bore.
A multiplicity of ways of utilising the screw are provided if the screw body is provided with an internal thread. In particular thereby, a screw head that may be adapted for a particular application can then be utilised, for example, a spherical head for a polyaxial tension mechanism. It is expedient hereby, if such a screw head is provided with a channel which is connectable to the hollow channel in the screw body so as to enable the bone cement to be supplied to the hollow channel in this manner.
It is of particularly great advantage if the screw body comprises a coupling element for a bone cement applicator. Such a coupling element may, for example, be arranged in a screw head which is formed in one-piece with the screw body or is connected thereto, or, it may be arranged directly in the screw body. It is advantageous if the coupling element comprises a seating arrangement for a nozzle of a bone cement applicator so as to enable bone cement to be injected into the screw body in a simple manner.
An especially stable arrangement for the anchorage is obtained if a diameter of the screw body is greater than a pitch of the external thread. Furthermore, a highly stable anchorage will be obtained if a bone screw in accordance with the invention comprises approximately seven to twelve peripheral turns of the external thread.
It is advantageous if the external thread is constructed in such a manner that the bone screw is self-tapping so as to enable universal usage thereof in this manner.
The following description of preferred embodiments of the invention will serve for a more detailed explanation thereof in conjunction with the drawing.